NEW YORK -- In a symbolic breakthrough for antigun activists, a federal-court jury for the first time found that a group of handgun manufacturers marketed firearms negligently and were directly responsible for three shootings that led to deaths and severe injury.

The jury, ruling in a civil lawsuit filed against the industry by families of seven gunshot victims, said only one victim of a nonfatal shooting suffered damages of about $4 million. The jury then reduced the amount of damages he would receive to about $520,000,...